The three-dimensional structure of crystalline southern bean mosaic virus is being determined by means of X-ray diffraction. The virus has approximately a 300 angstrom units diameter, a molecular weight of 6.4 x 10 to the 6th power daltons, with a protein coat of 180 subunits each of 28,500 daltons. An 11 angstrom units resolution electron density map shows the protein subunit to have bilobal structure. Data collection to 3.5 angstrom units resolution of the native and two heavy atom derivatives has been completed and two further derivatives are under investigation. If the interpretation of the difference Patterson is successful, an averaged electron density map to 3.5 angstrom units resolution is likely to follow rapidly. BIBLIOGRAPHIC REFERENCES: Rossmann, M. G. 1976. The refinement of heavy-atom parameters in the presence of non-crystallographic symmetry. Acta Crystallogr. A32:774-777. Argos, P., M. G. Rossmann, G. C. Ford. 1976. Solving heavy atom derivatives using non-crystallographic symmetry. In Crystallographic Computing Techniques. Ahmed, F. R., Huml, K. & Sedlacek, B., eds. Munksgaard, Copenhagen. pp. 222-228.